The problem is the smoke and mirrors accounting that shops use when they present the argument that they lose $$ on fill.
They are not including their own use of their compressor to fill their training and rental tanks. For many shops, that is the majority of their compressor usage.
The idea that filling divers tanks for their standard fee results in a lose of money for the shop is BS.
I disagree. The compressor is an expense over a profit generator. It's biggest attraction is convenience to the shop as you noted and to attract divers to the store to move retail merchandise.